Erratum in

Abstract

Exosomes are small endosome-derived extracellular vesicles implicated in cell-cell communication and are secreted by living cells when multivesicular bodies (MVBs) fuse with the plasma membrane (PM). Current techniques to study exosome physiology are based on isolation procedures after secretion, precluding direct and dynamic insight into the mechanics of exosome biogenesis and the regulation of their release. In this study, we propose real-time visualization of MVB-PM fusion to overcome these limitations. We designed tetraspanin-based pH-sensitive optical reporters that detect MVB-PM fusion using live total internal reflection fluorescence and dynamic correlative light-electron microscopy. Quantitative analysis demonstrates that MVB-PM fusion frequency is reduced by depleting the target membrane SNAREs SNAP23 and syntaxin-4 but also can be induced in single cells by stimulation of the histamine H1 receptor (H1HR). Interestingly, activation of H1R1 in HeLa cells increases Ser110 phosphorylation of SNAP23, promoting MVB-PM fusion and the release of CD63-enriched exosomes. Using this single-cell resolution approach, we highlight the modulatory dynamics of MVB exocytosis that will help to increase our understanding of exosome physiology and identify druggable targets in exosome-associated pathologies.

CD63-pHluorin is sorted into acidic MVBs and released via exosomes. (a) Proposed model for the visualization of MVB–PM fusion: a pH-sensitive optical reporter (CD63-pHluorin) is quenched when facing the acidic lumen of the MVB. Upon fusion, low luminal pH is immediately neutralized, resulting in a sudden increase in fluorescent intensity. EC, extracellular. (b) Immunofluorescent colabeling of total CD63 (red) and CD63-pHluorin (green) in HeLa cells. PCC, Pearson’s correlation coefficient. (c) TIRF images of a CD63-pHluorin–expressing HeLa cell at normal and elevated intracellular pH (NH4Cl superfusion). On the right, a heat map revealing acidic vesicles close to the PM was obtained by subtracting the fluorescent intensity values of the normal pH from the high-pH condition. (d) EM images of an MVB close to the PM (left) and EVs aligning the PM (right) labeled with gold particles directed to GFP (10 nm) in CD63-pHluorin–expressing HeLa cells. (e) Imaging flow cytometry of the number of late endosomes per cell in a 2.5-µm optical section in CD63-pHluorin–expressing cells (left) or immunostaining against LAMP1 in nontransfected cells (right; n > 2,000 cells). (f) Volume distribution of endosomes based on analysis of whole-cell confocal scans (error bars represent SD; n = 3). The blue area accounts for 75% of the total number of endosomes and covers the 400–600-nm-diameter range. (g) Immunogold labeling on purified exosomes with gold particles (10 nm) coupled to anti-GFP antibody. (h) Western blotting analysis on untransfected (−) and CD63-pHluorin–transfected (+) cells and purified exosomes for total CD63 and GFP. (i) Example of a localized sudden increase in fluorescence at the PM before the event (1), during the event (2), and right before disappearance of the signal (3). (j) Left: total projection of fusion events (bright spots) over a time course of 3 min onto two cells (blue). Right: representative example of CD63-pHluorin–expressing HeLa cell. N, nucleus. Bars: (b, c, and j) 10 µm; (i) 2.5 µm. (k) Effect of incubation with GW4896 (5 µM; n ≥ 8 cells per condition) and nSMase-2 knockdown (n ≥ 22 cells per condition) on fusion activity in HeLa cells. *, P < 0.05; ***, P < 0.001 using Student’s two-tailed two-sample t test. Whiskers in the box plots represent 1.5 times the interquartile distance or the highest or lowest point, whichever is shorter. (l) Western blotting analysis on purified exosomes from GW4896- and control-treated HeLa cells for CD63 and CD81.

CD63-pHluorin fusion events are derived from MVBs. (a) Left three panels: live imaging of fusion events (indicated by white arrows) over a time course of 12 s onto one cell before the event (left), at the start of the event (middle), and right before fixation of the cell (3). Right: inset showing a magnification of the localized sudden increase in fluorescence at the PM (highlighted by a dashed line square) right before fixation. (b) Left: correlation of light microscopy signal of a fusion event observed by live imaging with EM pictures of the first section of the cell facing the coverslip (low magnification). Right: correlation of light microscopy signal with the first slice of the electron tomographic reconstruction of the first section of the cell facing the coverslip. The orange circle indicates the error range (167 nm) of the correlation performed by eC-CLEM. (c) 3D model of the electron tomographic reconstruction. The ER is depicted in light violet. Dense compartments are depicted in brown. The structure of interest is depicted in red and orange. (d) Bottom side view of the 3D model of the compartment of interest in its surroundings. The white arrow indicates the opening of the MVB where ILVs are released. (e) 3D model showing the MVB isolated from its environment. ILVs secreted through the opening of the MVB are depicted in white. (f) Top view of the secretory profile of the MVB that correlates with the fluorescence burst of the CD63-pHluorin fusion event.